Field of the Invention
The present invention relates to a multiple beam phased array antenna system. More particularly, the present invention relates to a broadband wide-angle multiple beam phased array antenna system with reduced number of components using wide-angle gradient index lenses each with multiple scannable beams.
Background of the Related Art
Phased arrays are a form of aperture antenna for electromagnetic waves that can be constructed to be low-profile, relatively lightweight, and can steer the resulting high-directivity beam of radio energy to point in a desired direction with electrical controls and no moving parts. A conventional phased array is a collection of closely-spaced (half-wavelength) individual radiating antennas or elements, where the same input signal is provided to each independent radiating element subject to a specified amplitude and a time or phase offset. The energy emitted from each of the radiating elements will then add constructively in a direction (or directions) determined by the time/phase offset configuration for each element. The individual antennas or radiating elements for such a phased array are designed such that the radiated energy angular distribution or pattern from each feed in the array mutual coupling environment, sometimes called the embedded element or scan element gain pattern, is distributed as uniformly as possible, subject to the physical limitations of the projected array aperture over a wide range of spatial angles, to enable the maximum antenna gain over the beam scanning angles. Examples of conventional phased arrays are described in U.S. Pat. Nos. 4,845,507, 5,283,587, and 5,457,465.
In comparison to other common methods of achieving high directivity radio beams, such as reflector antennas (parabolic or otherwise) and waveguide-based horn antennas, phased arrays offer many benefits. However, the cost and power consumption of an active phased array, namely one incorporating amplifiers at the elements for the reception and/or transmission functions, are proportional to the number of active feeds in the array. Accordingly, large, high-directivity phased arrays consume relatively large amounts of power and are very expensive to manufacture.
Phased arrays typically require that the entire aperture is filled with closely-spaced feeds to preserve performance over the beam steering range when using conventional approaches. Densely packing feeds (spaced approximately half of a wavelength at highest frequency of operation) is required to preserve aperture efficiency and eliminate grating lobes. Broadband phased arrays are constrained by the element spacing, aperture filling fraction requirements, and the types of circuits used for phase or time offset control, in addition to the bandwidth limitations of the radiating elements and the circuitry.
For example, an approximately square 65 cm 14.5 GHz Ku-band phased array that is required to steer its beam to about 70 degrees from the array normal or boresight would require more than 4000 elements, each with independent transmit (Tx)-and/or receive (Rx) modules, phase shifters or time delay circuits, and additional circuitry. All the elements must be powered whenever the terminal is operating, which introduces a substantial steady-state DC current requirement.
Every element or feed in an active phased array must be enabled for the array to operate, resulting in high power drain, e.g., 800 W or more for a 4000-element array, depending on the efficiency of the active modules. There is no ability to disable certain elements to reduce power consumption without dramatically impacting the array performance.
Various techniques have been developed in support of sparse arrays, where the element spacings can be as large as several wavelengths. Periodic arrays with large element spacings yield grating lobes, but appropriately choosing randomized locations for the elements breaks up the periodicity and can reduce the grating lobes. These arrays have found limited use, however, as the sparse nature of the elements leads to a reduced aperture efficiency, requiring a larger array footprint than is often desired. See Gregory, M. D., Namin, F. A. and Werner, D. H., 2013. “Exploiting rotational symmetry for the design of ultra-wideband planar phased array layouts.” IEEE Transactions on Antennas and Propagation, 61(1), pp. 176-184, which is hereby incorporated by reference.
Another way to limit the effect of grating lobes is by using highly-directivity array elements, because the total array pattern is the product of the array factor, i.e. the pattern of an array of isotropic elements, and the element gain pattern. If the element pattern is very directive, this product suppresses most of the grating lobes outside the main beam region. An example is the Very Large Array (VLA). The VLA consists of many large, gimballed reflector antennas forming a very sparse array of highly directive elements (the reflectors), each with a narrow element pencil beam which dramatically reduces the magnitude of the sidelobes in the total radiation pattern from the array. See P. J. Napier, A. R. Thompson and R. D. Ekers, “The very large array: Design and performance of a modern synthesis radio telescope.” Proceedings of the IEEE, vol. 71, no. 11, pp. 1295-1320, November 1983; and www.vla.nrao.edu/, which is hereby incorporated by reference.